The present invention relates to an apparatus for taking samples by thermodesorption of components that are bound to solid absorbents, wherein
(a) the absorbents are contained in a sample tube; and
(b) the compounds are desorbed into a capillary column of a gas chromatograph by means of a stream of carrier gas and by way of an injector accommodating the sample tube.
The present invention relates particularly to an apparatus for injecting into the capillary column of a gas chromatograph trace substances which have been collected in sample tubes by adsorption from air or water.
Air and water are analyzed for their content of organic trace substances by pumping a stream of the medium to be examined over an adsorber pack on which the trace substances are collected and from which they are separated again by heating or elution with a solvent. The quantity of the trace substance, that is available for analysis however, is limited by the pump hold by the electrical power available for pumping. It is therefore desirable to inject the largest possible percentage of the collected trace substances into the capillary column of the gas chromatograph. Extracts cannot be evaporated to less than 100 .mu.l if errors due to loss of the trace substance are to be avoided. However, injecting such small solvent quantities is very difficult. If liquid samples are injected into the capillary column of a gas chromatograph, one must therefore accept that only a small portion of the extracts, at most 10 .mu.l, can be transferred to a capillary column, thus requiring large sample quantities.
The thermosdesorption technique has been developed to transfer the trace substances collected with the aid of a solid adsorbent into the capillary columm of a gas chromatograph. All varieties of this technique have in common that the charged absorber tubes are inserted into a heated injection chamber through which flows a carrier gas and which is sealed off from the atmosphere so that the adsorbed substances can be transferred to a capillary without losses.
One prior art device is composed of a quartz sample tube having an enclosed cooling trap. The sample tube is filled with 200 mg TENAX resin from which the substances are desorbed by heating, whereupon they are frozen out in a subsequent quartz capillary by cooling with nitrogen. TENAX is a brand of porous material based on a polymer of 2,6-diphenyl-p-phenylene oxide. The desorbed components are thus concentrated into a narrow band. This desorption unit can also be used as a sample injection device for larger volumes of liquid samples which are collected with the aid of a small precolumn.
If wall influences do not matter, the precolumn may also be made of metal. Such a column, for example, has a length of 16 cm and is made of 1/8" high-grade steel filled with TENAX GC 60/80 mesh. It is connected in an electric circuit as a resistor to effect resistive heating or is heated by means of a portable furnace.
If the substances are quickly released by heating, freezing may be omitted. For example, a commercially available device, operating with microwave heating, causes trace substances that are adsorbed by activated carbon, for example various ethers or Diesel fuel, to be desorbed. However, this method is limited to adsorption agents which can be heated by microwaves. Many polymers are not heated enough by such a device. For example, the adsorption agent TENAX, which is particularly suitable for collecting less volatile compounds, cannot be heated sufficiently with this device.
Many prior art injection devices contain heated valves, desorption furnaces or heated conduits. With the simpler devices, in which the adsorption sample tube is inserted into the injector, sample losses must be expected due to incomplete desorption of less volatile compounds.
Although, due to its thermal stability. TENAX is also suitable for desorption at higher temperatures, the task of transferring compounds having a higher boiling point to the capillary column by heating is accomplished only incompletely by the prior art devices.
Tests made with a commercially available thermodesorption device show that less volatile substances such as hexachlorobenzene and polychlorinated biphenyls can be transferred to the capillary column of the gas chromatograph only with considerable losses even if the carrier gas split connection is closed.